1. Field of the Invention
The present invention relates to a compact antenna apparatus for use in mobile communication equipment or the like.
2. Description of the Related Art
As small-sized antenna apparatuses for use in mobile communication equipment or the like, a variety of constructions have hitherto been proposed and used. As one well-known example of such small-sized antenna apparatuses, an inverted F antenna will be described below with reference to a plan view shown in FIG. 9.
In FIG. 9, reference numeral 11 represents a radiating element (driven element) composed of a radiating conductor portion 11a, a shorting conductor portion 11b, and a feeding portion 11c; 12 represents a grounding plate; and 13 represents a feeding point for the feeding portion 11c of the radiating element 11. The inverted F antenna having such a configuration has succeeded in reducing the size by bending a radiating element of a quarter-wavelength monopole antenna. Another feature of the inverted F antenna is that impedance matching can be achieved between the radiating element 11a and a feeding line (not shown) to be connected to the feeding point 13, such as a coaxial line.
A helical antenna can be taken up as another example with which compactness can be achieved. FIG. 10 is a plan view showing one example of a helical antenna. In the figure, reference numeral 14 represents a radiating element (driven element) composed of a quarter-wavelength conductor wire wound in a helical fashion; 15 represents a grounding plate; and 16 represents a feeding point for the radiating element 14. The helical antenna having such a configuration is known as a compact antenna which incurs less disturbance in directivity characteristics and excels in VSWR (Voltage Standing Wave Ratio) characteristics.
Recently, in keeping with rapid prevalence and advancement of mobile communication equipment, miniaturization has come to be increasingly demanded of the mobile communication equipment, and compact size and narrow mounting area are accordingly being demanded of an antenna for use in such equipment.
However, the structure of the inverted F antenna shown in FIG. 9 is contrary to the trend toward further miniaturization. This is because the inverted F antenna requires a considerable length equal to one quarter of wavelength, for example, needs to have a length so long as 9 cm at a frequency of 800 MHz, and is thus too large to be mounted in a small-sized mobile communication equipment.
Moreover, in general, miniaturization of an antenna gives rise to a problem of a gain being lower, as well as a problem of a bandwidth being narrower. In the helical antenna shown in FIG. 10, its electrical volume is reduced by confinement of magnetic energy. Therefore, miniaturization of such a helical antenna leads to a sharp decrease in bandwidth, thus making it extremely difficult to achieve a wide bandwidth even if a matching circuit is employed.
Another problem with the helical antenna is that there occurs relatively large conductor loss due to electric current flowing above the radiating element 14 made of a helical conductor line. When used in increasingly smaller and higher-frequency mobile communication equipment, this problem may lead to a decrease in the antenna gain.